As a wireless communication system using a 5 gigahertz [GHz] band, a wireless communication system of the IEEE 802.11a standard is known. In this wireless communication system, a throughput of a maximum of 54 megabits per second [Mbps] is realized using an orthogonal frequency division multiplexing (OFDM) modulation scheme, which is one of technologies for stabilizing characteristics in a multipath fading environment (for example, see Non-Patent Document 1).
However, the throughput here refers to a throughput on a physical layer. Because transmission efficiency in a medium access control (MAC) layer is actually about 50 to 70%, an upper-limit value of the actual throughput is about 30 [Mbps].
In addition, in IEEE 802.11n, single user-multiple input multiple output (SU-MIMO) technology capable of achieving spatial multiplexing using the same frequency channels at the same time through a plurality of antennas is known. In addition, it is possible to realize a transmission speed of a maximum of 600 Mbps through technology simultaneously using two 20 [megahertz (MHz)] frequency channels, which have been heretofore used individually, as a frequency channel of 40 [MHz] as technology aimed at realization of high-speed communication (for example, see Non-Patent Document 1).
In addition, frame aggregation technology for aggregating a plurality of frames and transmitting the aggregated frames is known. In addition, technology for improving the efficiency of data transmission by reducing an overhead of a control signal using a block acknowledge (block ACK) signal is known. In IEEE 802.11n, the realization of high-speed communication through these technologies is aimed at and it is possible to realize a transmission speed of a maximum of 600 [Mbps](see Non-Patent Document 1).
In addition, in the SU-MIMO, there is a method in which a data transmission station performs beamforming using propagation channel information (communication path information (CSI: Channel State Information) acquired in advance to improve characteristics. In addition, the IEEE 802.11ac standard currently being standardized is aimed at the realization of higher-speed wireless communication than in IEEE 802.11n through communication technology simultaneously using four 20 [MHz] frequency channels as a frequency channel of 80 [MHz]. In addition, the IEEE 802.11ac standard is aimed at the realization of higher-speed wireless communication than in IEEE 802.11n through multiuser MIMO (MU-MIMO) technology for performing communication with a plurality of wireless stations at the same time using the same frequency channels.
Hereinafter, the direction from the data transmission station (base station apparatus) to a data reception station (terminal station apparatus) is referred to as a “downlink”. In addition, the direction from the data reception station to the data transmission station is referred to as an “uplink”.
In the MU-MIMO, it is possible to calculate transmission weight values (transmission weights) capable of suppressing interference between data reception stations using downlink propagation channel information acquired in advance and realize high-speed communication by performing transmission using the transmission weight values (see Non-Patent Document 2). Here, the downlink represents a link from a data transmission station to a data reception station and the uplink represents a link from the data reception station to the data transmission station. As a method in which the data transmission station acquires the downlink propagation channel information, there is a method (terminal station estimation method) in which the data reception station estimates propagation channel information of the downlink and notifies the data transmission station of the estimated propagation channel information of the downlink. In addition, as another method for acquiring the propagation channel information of the downlink, there is a method (base station estimation method) in which the data reception station transmits a signal to the data transmission station and the data transmission station estimates the propagation channel information of the downlink based on the signal received from the data reception station.
The terminal station estimation method includes a step of transmitting a known signal for estimating propagation channel information from a data transmission station to a desired data reception station; a step of estimating, by the data reception station, the propagation channel information of a downlink from the difference between a received signal and the known signal; and a step of notifying, by the data reception station, the data transmission station of the propagation channel information of the downlink.
Hereinafter, a signal which is known and is used for estimating the propagation channel information is referred to as a “known signal”.
On the other hand, the base station estimation method includes a step of transmitting a known signal for estimating propagation channel information from a data reception station to a data transmission station; a step of estimating, by the data transmission station, the propagation channel information of an uplink from the difference between a received signal and the known signal; and a step of calibrating, by the data transmission station, the propagation channel information of the uplink to estimate propagation channel information of a downlink.
The base station estimation method is an efficient estimation method because the notification of the downlink propagation channel information is unnecessary.
The former acquisition method (terminal station estimation method) is referred to as explicit feedback (EFB) beamforming (see Clause 20.3.12.3 in Non-Patent Document 3). The explicit feedback beamforming is adopted in the IEEE 802.11ac standard (see Non-Patent Document 4).
However, in this propagation channel information estimation method, the data reception station provides the notification of the propagation channel information. Therefore, there is a problem in that an additional communication time for providing the notification is necessary and effective throughput is degraded.
In addition the propagation channel information changes with the movement of the data transmission station and the data reception station and a change in a surrounding environment. Therefore, when the communication time is greater than or equal to a predetermined time, there is a problem in that it is difficult to form transmission weight values capable of suppressing interference between data reception stations because the time at which the estimated propagation channel information is used is different from the time at which data is actually transmitted via the propagation channel, and consequently MU-MIMO transmission is difficult.
In order to solve these problems, in a time division communication system using the same frequency band in a downlink data transmission (data transmission from the data transmission station to the data reception station) and an uplink data transmission (data transmission from the data reception station to the data transmission station), transmission weight values of the downlink data transmission are calculated based on propagation channel information of the uplink data transmission. The time division communication system improves effective throughput by omitting the notification of propagation channel information using implicit feedback beamforming (IFB) technology (see Clause. 20.3.12.2 in Non-Patent Document 3) in which the calculated transmission weight values are used in the downlink data transmission.